The present invention relates to a process for the production of polymer materials.
Rein et al., in PCT Application No. WO 97/11037, which is incorporated by reference for all purposes as if fully set forth herein, describe a process for producing a polyolefin composite material from an assembly of polyolefin fibers by treating the assembly with a solvent such as xylene, or with a solution of the polyolefin, to swell the surficial layers of the fibers, growing and crystallizing "brush layers" on the fiber surfaces, and then heating and compressing the fibers. Preferably, the fibers are first placed under tension, the swelling of the surficial layers is effected at a temperature greater than that needed to melt the unloaded fibers but less than that needed to melt the loaded fibers, and the brush layers are grown and crystallized at a lower temperature.
Harpell et al., in European Patent Application 0 116 845, describe a process for transforming a network of high molecular weight polyethylene fibers into polyethylene articles, by the simultaneous application of a temperature between 100.degree. C. and 160.degree. C. and high pressure. The pressure is applied long enough to attain the desired degree of fiber coalescence, from simply causing adjacent fibers to adhere, to obtaining a film-like article which is substantially free of voids.
Ward et al., in U.S. Pat. No. 5,628,946, which is incorporated by reference for all purposes as if fully set forth herein, describe a process for producing a polymer sheet. An assembly of oriented polymer fibers is compressed at a contact pressure sufficient to hold the fibers in mutual intimate contact and then heated to a temperature sufficient to induce partial melting of the fibers. The melt fills the voids between the fibers. The assembly then is maintained at that temperature while being compressed at a still higher pressure to form the final product. In some cases, for example when the polymer fibers are made of highly oriented gel spun polyethylene, there is a tradeoff in the final product between high strength in the direction of fiber alignment, obtained by only limited partial melting, and high strength transverse to the direction of fiber alignment, obtained by more extensive partial melting.